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Middleware is a logic chain between the client and a Vapor route handler. It allows you to make operations on incoming requests before they get to the route handler, and on outgoing responses before they go to the client.

Configuration, and ErrorMiddleware

Middleware is registered in your configure.swift file. ErrorMiddleware is a very common example; it will take a thrown error in your software and convert it to a legible HTTP response code.

var middlewares = MiddlewareConfig()
// etc.

You will often run several middlewares in a single project. These middlewares are stacked up, and then registered together. The order in which middleware are listed can sometimes matter (see CORSMiddleware below).


FileMiddleware enables the serving of assets from the Public folder of your project to the client. You might include static files like stylesheets or bitmap images here.

var middlewares = MiddlewareConfig()

Now that the FileMiddleware is registered, a file like “Public/images/logo.png” can be linked from a Leaf template as <img src="/images/logo.png"/>.


Cross-origin resource sharing (CORS) is a mechanism that allows restricted resources on a web page to be requested from another domain outside the domain from which the first resource was served. REST APIs built in Vapor will require a CORS policy in order to safely return requests to modern web browsers.

An example configuration could look something like this:

var middlewares = MiddlewareConfig()
let corsConfiguration = CORSMiddleware.Configuration(
    allowedOrigin: .all,
    allowedMethods: [.GET, .POST, .PUT, .OPTIONS, .DELETE, .PATCH],
    allowedHeaders: [.accept, .authorization, .contentType, .origin, .xRequestedWith, .userAgent, .accessControlAllowOrigin]
let corsMiddleware = CORSMiddleware(configuration: corsConfiguration)

Given that thrown errors are immediately returned to the client, the CORSMiddleware must be listed before the ErrorMiddleware; otherwise the HTTP error response will be returned without CORS headers, and cannot be read by the browser.

Authentication and Sessions Middleware

The Vapor Auth package has middlewares that can do basic user validation, token validation, and manage sessions. See the Auth documentation for an outline of the AuthMiddleware.

Middleware API

Information on how middleware works and authoring custom middleware can be found in the Vapor API Documentation.